[lldb] Fix out of bounds read in DataExtractor::GetCStr and add unit test that function.

Summary:
The `if (*cstr_end == '\0')` in the previous code checked if the previous loop terminated because it
found a null terminator or because it reached the end of the data. However, in the case that we hit
the end of the data before finding a null terminator, `cstr_end` points behind the last byte in our
data and `*cstr_end` reads the memory behind the array (which may be uninitialised)

This patch just rewrites that function use `std::find` and adds the relevant unit tests.

Reviewers: labath

Reviewed By: labath

Subscribers: abidh, JDevlieghere, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68773

llvm-svn: 374311